Physiological Significance of NAD Kinases in Cyanobacteria.

Yuuma Ishikawa,Maki Kawai-Yamada

doi:10.3389/fpls.2019.00847

Abstract

Unicellular cyanobacteria are thought to be the evolutionary ancestors of plant chloroplasts and are widely used both for chemical production and as model organisms in studies of photosynthesis. Although most research focused on increasing reducing power (that is, NADPH) as target of metabolic engineering, the physiological roles of NAD(P)(H) in cyanobacteria poorly understood. In cyanobacteria such as the model species Synechocystis sp. PCC 6803, most metabolic pathways share a single compartment. This complex metabolism raises the question of how cyanobacteria control the amounts of the redox pairs NADH/NAD+ and NADPH/NADP+ in the cyanobacterial metabolic pathways. For example, photosynthetic and respiratory electron transport chains share several redox components in the thylakoid lumen, including plastoquinone, cytochrome b6f (cyt b6f), and the redox carriers plastocyanin and cytochrome c6. In the case of photosynthesis, NADP+ acts as an important electron mediator on the acceptor-side of photosystem I (PSI) in the linear electron chain as well as in the plant chloroplast. Meanwhile, in respiration, most electrons derived from NADPH and NADH are transferred by NAD(P)H dehydrogenases. Therefore, it is expected that Synechocystis employs unique NAD(P)(H) -pool control mechanisms to regulate the mixed metabolic systems involved in photosynthesis and respiration. This review article summarizes the current state of knowledge of NAD(P)(H) metabolism in Synechocystis. In particular, we focus on the physiological function in Synechocystis of NAD kinase, the enzyme that phosphorylates NAD+ to NADP+.

Highlights

nicotinamide adenine dinucleotide (NAD)(P)(H) are important electron carriers, employed by all living cells in energy conversion
It is thought that the presence of the non-phosphorylated forms (NAD+ and NADH) and phosphorylated forms (NADP+ and NADPH) make it possible for multiple redox reactions to coexist simultaneously within the cell (Alberts et al, 2002)
Under photoheterotrophic conditions (e.g., in medium containing glucose and 3-(3,4dichlorophenyl)-1,1-demethylurea (DCMU), a compound that inhibits photosynthesis), and under low-light conditions in the presence of glucose, cyanobacteria utilize medium-supplied glucose as both an energy and carbon source, and generate NADPH through the oxidative pentose phosphate pathway to compensate for photosynthesis-derived NADPH (Narainsamy et al, 2013; Nakajima et al, 2014; Ueda et al, 2018). Based on these unique metabolic characteristics, cyanobacteria provide a unique tool for examining several relevant topics, including: (1) How do cyanobacteria control the amount of the redox pairs NADH/NAD+ and NADPH/NADP+ in mixed metabolism in the single compartment? (2) Which metabolic pathway is controlled by NAD(P)(H) under the various growth conditions in cyanobacteria?

Summary

Introduction

NAD(P)(H) are important electron carriers, employed by all living cells in energy conversion. (2) Which metabolic pathway is controlled by NAD(P)(H) under the various growth conditions in cyanobacteria? Sll1415-deficient mutant, one of the NADK-deficient mutants in Synechocystis, and pntAB-deficient mutant showed the similar phenotype under the day-night rhythm in the presence of glucose and the mixotrophic condition (20 μE/m2/s) (Ishikawa et al, 2016, 2019; Kamarainen et al, 2017). PCC 6803 harbors two NADK-encoding genes (sll1415 and slr0400).

Results

Conclusion